1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The patterned beam is projected onto the target potion of the substrate by a projection system including a series of lens components. In one arrangement, an array of lenses (also referred to as a lens array) is positioned adjacent the substrate with each lens in the array of lenses being arranged to focus a respective part of the patterned beam onto the substrate as a single illumination “dot.” This arrangement is normally referred to as a microlens array or MLA systems. In order for the pattern of dots projected by the lens array to be scanned across the substrate, relative displacement as between the microlens array and the substrate is required. Generally, but not necessarily, this is achieved by displacing the substrate beneath a static lens array.
The resolution of the pattern projected by the mircolens array, that is the smallest dimension of a feature which can be represented in the pattern, is proportional to the wavelength of the patterned beam, and inversely proportional to the numerical aperture (NA) of the lens system. That smallest dimension is generally referred to as the critical dimension (CD), and typically may be in the range of 1 to 2 micrometers, or smaller.
In an MLA system, the numerical aperture is a function of the angle subtended at the substrate by radiation focused onto the substrate by a respective lens in the lens array. That subtended angle is sometimes referred to as the “opening angle.” Thus, for a given numerical aperture, (e.g., typically in the range of about 0.06 to 0.25), the required minimum diameter of each lens in the lens array is a function of the spacing between the lens array and the substrate onto which the lens array projects the patterned beam. That spacing is generally referred to as the “free working distance.” The greater the free working distance, the greater the diameter of each lens in the lens array, and therefore the greater the minimum pitch of lenses in the array. Achieving higher numerical apertures requires either larger lenses (and a resultant larger pitch), or smaller free working distance, or a combination of the two.
Typically, the free working distance between the lens array and the substrate is a few hundred micrometers, for example in the range 200 to 800 micrometers. In MLAs it is desirable to provide arrays of 256×256 to 1024×1024 lenses in an area of 127×127 mm2 (e.g., the effective area of a 200 mm substrate). Given such dimensions, a maximum possible lens pitch in the array is typically in the range 124 micrometers (127 mm/1024) to 496 micrometers (127 mm/256).
In order to achieve higher resolutions, it is desirable to operate with relatively small free working distances. In an MLA, the lens array may be displaceable towards and away from the substrate to maintain the actual array/substrate spacing at the designed free working distance. In normal circumstances, there is little risk of potentially damaging contact occurring between the lens array and the substrate.
In flat panel display technology environments, a substrate may be large, for example a large glass panel of the order of 2 meters square, and will generally be thin, for example of the order of 700 micrometers. With such large thin panels there is a real risk of debris, for example particles of glass or other materials, being present on the substrate. For instance, if the substrate has been coated with a resist prior to its exposure in the lithographic apparatus, it can be difficult to clean all such contaminants from the substrate. If a particle that is larger than the free working distance between the substrate and the lens array is present on the substrate, it will contact the lens array as the substrate is advanced beneath the lens array, and as a result, will cause damage to the substrate, leading generally to the rejection of the damaged substrate. Of greater significance is that such a particle collision may well scratch or cause other damage to the lenses of the array. Such an event would necessitate replacement of the lens array, which is a time consuming and expensive maintenance issue.
Therefore, what is needed is a lithographic system and method that eliminate or substantially reduce damage to lens arrays by contaminants.